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Quantum 2.0 Conference and Exhibition

Quantum-Enhanced THz Time-Domain Spectroscopy

Dionysis Adamou, Lennart Hirsch, Taylor Shields, Seungjin Yoon, Adetunmise C. Dada, Jonathan M R Weaver, Daniele Faccio, Marco Peccianti, Lucia Caspani, Matteo Clerici

Published Jan 1, 2025• 0 citations
Publisher

Abstract

We investigate the possibility of improving time-domain spectroscopy of Terahertz radiation using quantum states of light. We experimentally demonstrate that using quantum-correlated pulsed twin beams as the probe field significantly enhances the detection sensitivity.
DOI
10.1364/quantum.2025.qth2c.5

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Time-domain spectroscopy (TDS) is among the most sensitive techniques used to study the properties of materials, structures and biological samples [1]. It is usually implemented using electro-optical sampling in X(2) crystals, where the polarization state of a short probe pulse is modified by the local electric field of the radiation to be measured. Such a polarization change is measured by a balanced detector as the delay between the probe pulse and the radiation under measurement varies, allowing the electric field of the latter to be sampled in the time domain.
The time-resolved detection of mid- to far-infrared electric fields absorbed and emitted by molecules is among the most sensitive spectroscopic approaches and has the potential to transform sensing in fields such as security screening, quality control, and medical diagnostics. However, the sensitivity of the standard detection approach, which relies on encoding the far-infrared electric field into amplitude modulation of a visible or near-infrared probe laser pulse, is limited by the shot noise of the latter. This constraint cannot be overcome without using a quantum resource. Here, we show that this constraint can be overcome using a two-mode squeezed state. Quantum-correlated ultrashort pulses, generated by parametric down-conversion, enhance the sensitivity of far-infrared detection beyond the classical limit, achieving a twofold reduction in measured noise. This advancement paves the way for further development of ultrafast quantum metrology, moving toward quantum-enhanced time-resolved electric field spectroscopy with sensitivities beyond the standard quantum limit.
In conventional balanced-detection measurements, sensitivity can generally be improved by increasing the optical power. However, in many systems this approach reaches an upper bound beyond which increasing the probe power can damage the sample or degrade the measurement. In THz time-domain spectroscopy, this bound is set by nonlinear noise and measurement back-action. When this ceiling is reached by classical techniques, a potential avenue to improve the signal-to-noise ratio is by using quantum metrological resources. This work reports the development of a bright, ultrafast twin-beam source, exhibiting more than 6 dB of noise suppression up to microwatt optical power levels, together with an ultra-low-noise balanced photoreceiver with sub-shot-noise performance. Integrating these technologies into a THz-TDS system developed in collaboration with colleagues we demonstrate, for the first time to our knowledge, a measurable enhancement in sensitivity by harnessing quantum correlations when compared with a classical source under identical conditions. Although the implemented THz-TDS setup deviates from conventional polarimetric schemes, which typically achieve higher sensitivity, these results show that quantum methods can provide a sensitivity enhancement in field-resolved THz spectroscopy. Beyond this demonstration, the developed twin-beam source has clear potential for wider use as its brightness, stability and correlation strength make it an attractive candidate for ultrafast, differential transient absorption measurements.
Electro-optical sampling of Terahertz fields with ultrashort pulsed probes is a well-established approach for directly measuring the electric field of THz radiation. This technique usually relies on balanced detection to record the optical phase shift brought by THz-induced birefringence. The sensitivity of electro-optical sampling is, therefore, limited by the shot noise of the probe pulse, and improvements could be achieved using quantum metrology approaches using, e.g., NOON states for Heisenberg-limited phase estimation. We report on our experiments on THz electro-optical sampling using single-photon detectors and a weak squeezed vacuum field as the optical probe. Our approach achieves field sensitivity limited by the probe state statistical properties using phase-locked single-photon detectors and paves the way for further studies targeting quantum-enhanced THz sensing.